The present invention relates to an apparatus for autogenous grinding of a homogeneous or heterogeneous coarse crushed mineral material in a rotary drum mill with a screening wall arranged inside the drum at its discharge end. In autogenous grinding, the grinding charge active for grinding in the mill or mills comes from the mineral material itself, a subsequent grinding step being supplied with a grinding charge from the grinding charge of a preceding mill by selective discharge from the latter.
The present invention also relates to a method of comminuting a heterogeneous, coarse lump or coarsely crushed mineral material. In the expression "mineral material" there may also be included "industrial minerals" and together or individually these kinds of materials are also called "ore" or "ores" in the following description.
In dressing mineral material, e.g ore, for the selective or collective extraction of valuable substances or material components, these operations or processes are preceded by a mechanical pulverization of the material such that the valuable components are separated from each other. By using known separation processes the valuable components can be segregated. The mechanical pulverization or comminution usually starts already al the extraction of ore from a mine or surface digging. It is also normal here to crush the ore in one or more steps before final grinding to a fine material size suitable for the process. Depending on the properties of the ore , as well as the grinding technique which is to be used, the mineral material is crushed to a maximum lump size varying between about 500-100 mm.
The techniques dominating in grinding operations are:
autogenous, in which grinding is done by utilizing grinding bodies from the material itself;
semi-autogenous, in which the grinding bodies form the material itself are partially substituted by steel balls;
conventional, in which grinding is done exclusively by steel rod or ball grinding bodies.
Historically, the conventional grinding technique has been predominant, and it is preceded by extensive crushing of the mineral material or ore before grinding, which gives a stable grinding process, due to the grinding charge being homogeneous in weight and composition. However, the conventional technique is the most outstandingly expensive of the grinding techniques mentioned, because of high investment and operational costs.
Particularly with lean ores and high production, the autogenous technique is to be preferred to the others, due to lower costs and the absence of foreign material in the shape of grinding bodies, and also because the conventional technique gives rise to Fe ions in the comminution, which sometimes results in a poorer yield in flotation subsequent to grinding.
By autogenous techniques is intended the situation where the grinding charge essentially comprises grinding bodies from the mineral material itself, but that both external competent mineral material and steel balls or other material can be added as substitutes when there is a lack of supply of grinding bodies from the material itself.
A very usual situation is, however, that the mineral material contains too great an amount of difficultly ground, so-called "over-competent" material. which has a very negative effect on grinding capacity, and thereby on both grinding result and cost, and this situation must be dealt with if required profitability is to be attained with autogenous grinding.
It is known from the state of the art, e g. Swedish patent document SE-PS 7909921-4, that in grinding mineral material there is always material having different properies to be taken into consideration, and the known technique giving the best technical/economical result is selected, with due regard to the material properties.
Within the field in question, it is known that, depending on its comminution properties and /or its "grinding resistance", the mineral material has different properties, which form the autogenous grinding aspect can normally be divided into three different competence ranges, or be defined as:
1). Competent, i.e ores having sufficient mechanical strength to form an active grinding charge by themselves, and are thus suitable for autogenous grinding;
2). incompetent, i.e. ores requiring an addition of foreign bodies, e.g. steel balls, to enable their comminution, and which are thus suitable for semi-autogenous grinding or conventional grinding; and
3). over-competent, i.e. ores which have very high mechanical strength, where their comminution in an autogenous grinding process requires a very high energy input, and which are thus suitable for conventional or semi-autogenous grinding.
Within this technological field it is further known that comminution of a mineral material with the aid of autogenous grinding techniques lakes place, generally speaking, in three different ways, namely:
1). By impact, i.e. shock on falling onto a substructure, or against the material itself, which is saving in energy;
2). by attrition, which is the most usual way in crushing, in rod or ball mills or in autogenous mills under favorable conditions, and means that small pieces are comminuted by pressure and shearing between larger pieces and/or between surfaces under pressure, this way resulting in a high saving in energy and being striven after in autogenous grinding processes;
3). by abrasion, i.e. comminution by the surfaces of material pieces being rubbed/worn against each other, which normally requires a large amount of energy and often gives an unsuitable, uncontrollable ground product, and should therefore be avoided as far as possible.
With "over-competent" material there is formed an excess of so-called critical material, i.e. difficultly ground fractions. This and increasing over-competence result in that the grinding space of the mill is successively filled by critical f actions, with resulting rapidly decreasing grinding capacity.
In the above mentioned Swedish patent examples are given of a technique that also substantially improved autogenous grinding of material having a pronounced over-competence for certain intermediately competent materials and especially such that are clearly heterogeneous, this known technique gives unnecessarily high plant and operating costs, since in such cases more force than necessary is used.